exam Maths Puzzle Numerical Ability

I dont understand this:

Kendra invested $850, part of it at 9% and part of it at 12%. If her interest for one year was $81 how much did she invest at each rate?

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exam Other Question
Combine like terms:
x + 6x2 - 4 - x2 - 3 + 9x + 8 I need help and this not from text book 3X^2 - 3X - 60. I get 3x(3-20) Thank you